You're right, Chelios is probably the best example. He wasn't great at the end of his career with the habs but by then he was too old and extended his stay in the NHL, but it's still a great example. Jagr is a great example in today's NHL too...I guess the game has slowly changed in a way where top line forwards can't be the most punishing players on their team aside from the select few who can handle both those jobs (Iginla-type players). I'm not too sure about off-ice/off-season diets though, didn't really think a few months away from hockey could have that big of an effect on people.
Seems like the most plausible explanation, since it was common-place in the 80s and early 90s for guys to retire at 32/33, and we all know those guys weren't really taking care of themselves. At least not at the level guys take care of themselves today. There were obvious exceptions for mega-elite talents who, even after slowing down, were able to rely on other parts of their skillset to dominate (Lemieux).
Normally though, when you hear about guys who prolong their careers, it's almost always the same story: they put the effort in in the gym. Gary Roberts, I guess, is probably an even better example of that than Chelios, and he really changed what people thought was possible for your non-elite player in his thirties.
But you rarely see steep Richards-type declines on guys with a pristine work ethic reputation, unless there are major injuries involved. The Kings even acknowledged that, prior to this year, Richards wasn't exactly in the best of shape. Hard not to connect those dots. It didn't help that Richards couldn't afford to lose a step.